1. Field of the Invention
The present invention relates generally to a visual recognition apparatus for use in, for example, a robot, and more particularly to an optical information processor for optically performing image processing or image recognition. The present invention also relates to a Fourier transform lens assembly for use in the optical information processor.
2. Description of the Prior Art
Recently, there have been strong demands in the field of image processing or image recognition towards a higher speed processing of a large number of pixels than that hitherto accomplished. To this end, the development of an optical information processor is being advanced by making use of a high-speed parallel operation function of light.
Japanese Laid-open Patent Publication (unexamined) No. 2-132412 discloses an optical information processor as shown in FIG. 1. In FIG. 1, reference numeral 20 denotes a TV camera, reference numeral 21 a first liquid crystal display for displaying an image picked up by the TV camera 20, reference numeral 22 a laser diode, reference numeral 23 a collimator lens for collimating light from the laser diode 22, and reference numeral 24 a first lens. The first liquid crystal display 21 is located on the front focal plane of the first lens 24. Reference numeral 25 denotes a second crystal liquid display located on the back focal plane of the first lens 24.
Furthermore, reference numeral 26 denotes a ROM (read-only memory), reference numeral 27 a second lens, and reference numeral 28 a photodetector. In the ROM 26 are written data of Fourier transform type computer-generated holograms calculated in advance with respect to a plurality of reference patterns, i.e., data of applied voltages corresponding to the transmittance of respective pixels on the second liquid crystal display 25. In calculating the Fourier transform type computer-generated holograms, a plurality of pixels on the second liquid crystal display 25 are rendered to be sampling points. The second liquid crystal display 25 and the photodetector 28 are located on the front focal plane and on the back focal plane of the second lens 27, respectively.
The optical information processor having the above-described construction operates as follows.
When an image of an object is picked up by the TV camera 20, it is displayed on the first liquid crystal display 21. The laser diode 22 applies to the first liquid crystal display 21 a coherent beam collimated by the collimator lens 23.
Because the first liquid crystal display 21 is located on the front focal plane of the first lens 24, a Fourier transform image of the object optically transformed by the first lens 24 is formed on the back focal plane of the first lens 24, i.e., on the second liquid crystal display 25. At this moment, upon input of the data stored in the ROM 26 into the second liquid crystal display 25, the transmittance of each of the pixels on the second liquid crystal display 25 is spatially modulated. As a result, each of the Fourier transform type computer-generated holograms of the specific reference patterns is displayed on the second liquid crystal display 25, which functions as an optical filter. Accordingly, on the second liquid crystal display 25, the Fourier transform image, which has optically been transformed by the first lens 24 from the input image of the object displayed on the first liquid crystal display 21, is superimposed on each of the Fourier transform images calculated in advance with respect to the specific reference patterns.
Furthermore, because the second liquid crystal display 25 is located on the front focal plane of the second lens 27, when the Fourier transform image of the object and that of a specific reference pattern coincide with each other, i.e., when both of them are indicative of the same object, a bright point appears on the back focal plane of the second lens 27 and is detected by the photodetector 28. In this way, an optical image processing is performed wherein an optical filter in the form of a computer-generated hologram displayed on the second liquid crystal display 25 functions as a matched filter.
The above optical information processor is disadvantageous in that the length of an optical path is elongated for the following reasons, thereby enlarging the size of the apparatus. If the wavelength of the laser diode 22, the pixel pitch of the first liquid crystal display 21, and the diameter of a Fourier transform image displayed on the second liquid crystal display 25 are .lambda., P, and D, respectively, the focal length (f) of the first lens 24 is given by f=D.cndot.P/.lambda.. When P=50 .mu.m, .lambda.=0.8 .mu.m, and D=60 mm, a lens of f=3,125 mm is required. Accordingly, as shown in FIG. 1, the distance between the first liquid crystal display 21 and the second liquid crystal display 25 is 2.cndot.f=6,250 mm. It must be said that this distance is extremely long.